In incomplete dominance both of the genes of an allelomorphic pair express themselves partially. One gene cannot suppress the expression of the other completely. Thus the pair is not as one dominant and other recessive. In such cases there is intermediate expression in F1 hybrids.
In four o' clock plant, (Mirabilis jalapa) there are two pure varieties; when one with red (RR) flowers is crossed with other with white (rr) flower, the F1 hybrids bear pink (Rr) flowers. When by selfing of F1 hybrids, F2 generation was raised, it showed; red (RR), pink(Rr) and white (rr) in 1:2:1 ratio. (Phenotypic and genotypic ratio is same). This means factors segregate and there is no mixing of the factors. The intermediate shade is produced due to incomplete dominance. Incomplete does not favour the blending theory of inheritance, though in F1 all are pink; both the parental traits, red and white reappear, each in 25% in F2 generation. Other similar example is Snapdragon, ( Antirrhinum majus ).